Resin-bonded abrasive articles such as grinding wheels are typically produced by blending discrete abrasive grain or grit particles with a liquid resin material and a powdered resin, and then pressing the mixture under appropriate thermal conditions. Other constituents can be included in the mixtures, e.g., fillers, curing agents, wetting agents, and various metal powders. An aging period which allows for solvation of the dry portion of the mixture with the liquid resin is usually required before pressing.
In the manufacture of abrasive articles it is necessary to bind the abrasive grain particles together so that the article may be molded and otherwise handled prior to the curing process. Heat is applied during the curing process to fix the article into the desired shape. The ideal temporary binder provides green strength to the uncured abrasive article, provides flexibility in scheduling of manufacturing, i.e., no aging step is needed; is useful in either a compression molding (cold press) or hot press operation, and does not cause irreversible agglomeration of the abrasive grain when the grain is stored prior to molding of the abrasive article. Green strength is important both in the removal of an uncured abrasive article from the mold and transfer of the article to facilities for curing the abrasive article, and in maintaining the integrity of the desired shape, particularly in precision grinding wheels.
As noted in U.S. Pat. No. 4,918,116 (Gardziella et al), phenol novolac resins have been used in organic solvent solutions for bonding abrasive articles. No temporary binder is needed prior to cure. Disadvantages of such a system include the easy ignitability of the solvents at high temperatures and waste disposal. While solvent-free modified novolac resins have been developed, these materials are quite expensive.
In addition to the difficulties involved with using certain phenol-novolac binders for making abrasive articles, manufacturers are sometimes faced with other production problems as well. For example, the use of liquid grain wetting agents such as liquid phenolic resin when preparing molding materials for abrasive wheels may result in an unstable molding mixture. Furthermore, the use of such a mixture may generate a large amount of dust, often a drawback on the manufacturing floor.
The dust and stability problems associated with using novolac binders appears to have been somewhat alleviated by the teachings set forth in Gardziella. This reference discloses the preparation of various molding materials, using specific phenol-novolac resins having a phenol-formaldehyde molar ratio of 1:0.2 to 1:0.35. As an example, abrasive discs are prepared by using heated corundum grains wetted with a hot melt of the specified phenol-novolac resins. After being blended at 140.degree. C. in a high-power mixer, the composition is cooled to 90.degree. C. and then further blended with a second novolac resin and a curing agent.
Gardziella limits his comments to "the high-temperature resistance molding materials for the production of hot-pressed abrasive discs." He does not address the cold pressing of abrasive articles. Based on other teachings in the art, presumably an organic binder, such as furfural, would be used as a temporary binder in cold pressing to permit molding and handling of the uncured abrasive article. In the past, organic solvents and other organic materials, such as furfural and alcohols which are compatible with phenolic resins and with rubber materials used to provide more flexible resins in abrasive articles, have been used as temporary binders.
Due to the increased attention given to environmental concerns, the use of organic solvents or other organic materials as temporary binders creates difficulties in manufacturing. Organic binders are undesirable in the air, water and solid waste effluent streams. They contribute to the volatile organic chemical content of the uncured abrasive article and, possibly, the cured article; to additional inventory controls required for organic solvents; and to landfill concerns arising from the disposal of used abrasive articles, such as wheel stubs. Organic materials tend to leach out of the used abrasive articles in landfills, thereby creating potential ground water contamination, soil contamination and other environmental and regulatory concerns.
Some environmental concerns are alleviated by the use of the phenol-novolac resin of the sort taught by Gardziella. In particular, these resins are characterized by an exceptionally low free phenol content, in the order of less than 0.5%.
It has now been discovered that water, an environmentally friendly solvent, is an excellent temporary binder for phenolic resin coated abrasive grain. Water provides excellent green strength to the uncured abrasive article, is useful in cold pressing operations, permits the reuse of abrasive grain mixtures and flexibility in manufacturing operations, and is entirely free of environmental concerns. The use of water as a temporary binder is particularly beneficial when done in combination with a low volatile organic chemical content resin, such as the phenol-novolac resin of Gardziella.
Furthermore, the final article must retain its functional properties. In the case of an abrasive wheel, the desirable properties include grindability and long working life. Water used as a temporary binder has no adverse effects on the final abrasive article.